Cathode ray tubes are used in a number of applications, including oscillography, computer data display, radar display, television, and electronic phototypesetting. Devices serving these application may be generally referred to as video display systems. Video display systems are also sources of electromagnetic radiation and typically must be shielded to reduce the emission of electromagnetic interference.
A cathode ray tube is driven by electrical signals which are generated by a video amplifier. These signals are carried from the video amplifier to the cathode ray tube by video amplifier wires. The voltage of the video amplifier signals may be at tens of kilovolts and the current at hundreds of thousands of microamperes. The video amplifier, and the video amplifier wires which carry these signals, emit electromagnetic radiation and must be shielded.
Electromagnetic radiation generated by the video amplifier is typically shielded by a conductive enclosure which surrounds the video amplifier. The enclosure is often formed from wire mesh or a metal casting. Because access to the enclosure must be provided to allow the placement of the video amplifier within the enclosure, the enclosure typically consists of multiple parts. One part may form a cavity into which the video amplifier is positioned, and another part may cover the cavity opening to complete the enclosure of the video amplifier.
The enclosure and video amplifier assembly must then be mechanically mounted to the video display system. To maintain the electromagnetic radiation shielding effectiveness of the enclosure, the enclosure must also be electrically connected to the shielding portion of the video display system. This mechanical and electrical mounting is often done by fastening the enclosure with numerous conductive fasteners to a conductive base plate. The fastening is often accomplished with metal screws or rivets, or by welding. These mounting methods are typically permanent, labor intensive, or both. Permanent mounting, such as by welding or riveting, prevents the servicing of the video amplifier after the video display system has been manufactured. Removable mounting, such as by sheet metal screws, is labor intensive and not well suited to an efficient manufacturing process.
Alternatively, the video amplifier enclosure may be mechanically mounted within the video display system without regard to electrical conductivity. This allows the enclosure to be mounted to a nonconductive portion of the video display system. The enclosure must then be electrically connected to the shielding portion of the video display system by a conductive member, such as a ribbon of braided wire. However, separate mechanical and electrical mounting of the video amplifier enclosure requires separate assembly steps. These additional assembly steps, which are typically performed manually, add to the time and cost to manufacture the video display system.
Video amplifier wires are commonly shielded by using shielded wire. Shielded wire typically includes a conductive core for carrying the signals, an insulating layer surrounding the conductive core, and a conductive shield surrounding the insulating layer. The conductive shield is often a weave of conductive wire which forms an encasing mesh around the insulating layer. Conductive foils and the like are also used for the conductive shield.
Shielded wire, while effective to shield electromagnetic radiation emitted by the signals in the wire, is bulkier and more expensive than unshielded wire. Shielded wire also increases the cost and complexity of the assembly process. The shielded wire is more difficult to handle than unshielded wire, and the shield of each wire must be attached so that it is electrically connected to the video amplifier shield.
Therefore, it is desirable to have an inexpensive device that can effectively shield electromagnetic interference emitted by video amplifier wires without requiring each video amplifier wire to be separately shielded. It is also desirable to incorporate this shielding function into the video amplifier shielding enclosure. Further, it is desirable to have an inexpensive device that can mechanically and electrically mount the shielding enclosure simply and inexpensively, and that the shielding enclosure be mountable without any tools or additional fasteners. It is further desirable to have a shielding enclosure formed from a single part for ease of manufacture and assembly.